1. Field of the Invention
The invention broadly relates to filter media and filter apparatus comprising such filter media. More particularly it concerns rigid filter media capable of withstanding heavy loads upon the top surface and that is well suited for use in dewatering sewage sludge.
2. Description of Prior Art
In the operation of a sewage treatment plant it is necessary to dewater sewage sludge as a step in disposing of it. In conventional operations, the sludge influent to the dewatering step will contain not over about 2% solids. In order to handle the sludge as a "dry" product in trucks for hauling to a disposal site, the sludge should contain at least about 8% solids.
The typical equipment for sludge dewatering is an open tank containing a bed of loose sand supported on a lower bed of gravel which in turn is supported upon the floor of the tank, usually made of concrete. The dewatered sludge builds up on the top of the sand layer as water drains through until the sludge layer contains about 6% solids. At such point, cracks start to occur in the layer and these grow as more water is removed. Depending upon the amount of time the sludge charge is allowed to remain in the tank, it can be dewatered up to 12-15% solids. At any point above about 6-8% solids content, the sludge is removed from the sand bed by shovels, back-hoes or other scraping or scooping equipment. Usually a thin layer of sludge will remain on the top of the sand bed and will hinder the dewatering of the next batch of influent. Rakes or scrapers are often used to remove this sludge residue, but this can result in loss of sand eventually requiring replacement of the sand bed. Furthermore, this sand bed type sludge dewatering is undesirably slow and requires large area filters to attain needed throughput.
Another conventional method of dewatering sludge uses contrifuges. This presents problems as to initial equipment costs and maintenance expenses.
Another type of sludge dewatering apparatus has used a rigid bed or plate of solid particles bound in a matrix of cured resin in place of the conventional sand bed. Plates of this type are discussed in our prior U.S. Pat. No. 4,208,288, the disclosure of which is incorporated herein by reference.